1. Field of the Invention
The present invention relates to a switching device for switching on and off by operation of an operation knob which swings, and more particularly to a waterproof-type switching device capable of preventing water from entering into its case.
2. Description of Related Art
FIG. 11 is a side view schematically showing a structure of a switching device 100 used in a window open/close system (power window system) of a vehicle. The switching device 100 has a case 1, and switches, a circuit board and other components which will be described later are accommodated inside the case 1. A seesaw-motion type operation knob 6 of a window open/close switch is operated to open and close a window of the vehicle. The operation knob 6 has an operation section 6a, a cap section 6b formed integrally with the operation section 6a, and holes 6c formed on the side walls of the cap section 6b. A cylinder 7 is provided on an upper surface 1e of the case 1, and a shaft 7a is formed integrally with the outer wall of the cylinder 7. The cap section 6b of the operation knob 6 is attached to the cylinder 7 from above, and the holes 6c of the operation knob 6 are brought into engagement with the shaft 7a of the cylinder 7. By this engagement, the operation knob 6 is supported by the cylinder 7 such that the operation knob 6 can swing around the shaft 7a. An operation knob 4 is an operation knob of a door lock switch, and is operated to lock the doors of the vehicle such that the doors cannot be opened, and to release the lock. The switching device 100 is attached to an arm rest (not shown) provided on the door of the driver's seat of the vehicle, and is covered by a cover 11 shown in FIG. 3 in areas other than the positions of the operation knobs.
FIG. 12 is a cross-sectional view showing a main part of the switching device 100 to which the cover 11 is attached. As apparent from the figure, the cylinder 7 is a hollow cylinder which is open to above and below to communicate with the inside of the case 1. A circuit board 14 is provided inside the case 1, and a switch 13 is packaged on the circuit board 14. The switch 13 is a switch for opening and closing a window, and is formed by a known slide switch. The switch 13 has an actuator 13a. An operation bar 12 connected to the operation knob 6 extends through a lower opening 7c of the cylinder 7 toward the inside of the case 1. A concave 12a is formed at the lower end of the operation bar 12. By engagement between the concave 12a and the actuator 13a of the switch 13, the motion of the operation knob 6 is transmitted through the operation bar 12 to the switch 13, and contacts equipped inside the switch 13 are switched between on and off in accordance with the operating position of the operation knob 6. A point Q is the rotation center of the operation knob 6, and corresponds to the position of the shaft 7a shown in FIG. 11. An opening 1a through which the finger is inserted to operate the operation section 6a of the operation knob 6 is formed on the cover 11. The operation section 6a is exposed through the opening 11a. 
A switching device having a similar structure as that of the switching device 100 explained above is shown in JP-A-8-180755 and JP-A-5-314864 (Patent References 1 and 2). JP-A-11-86662 (Patent Reference 3) discloses a switching device having a similar structure as above, which is waterproofed by surrounding soldered portions of electronic component terminals on a circuit board with side walls formed by rubber contact members without requiring coatings.
In the switching device 100 described above, the opening 11a of the cover 11 is sized large enough to prevent any trouble which may be caused when putting the finger on the operation section 6a of the operation knob 6 to lower or raise the operation section 6a. As a result, raindrops entering through the window which has been left open, beverage accidentally spilt in the vehicle compartment or the like (hereinafter collectively referred to as “water”) flow through the opening 11a toward the concave 11b in some cases as illustrated in FIG. 12. In this case, the water entering the concave 11b flows through a clearance 15 formed between a bottom wall 11f of the concave 11b and the cap section 6b of the operation knob 6 and through a clearance 16 formed between the bottom wall 11f and the case 1 to be discharged to the outside. However, when a large amount of water flows into the concave 11b, the water is not sufficiently discharged and flows through the clearance 15 into a space between the cap section 6b of the operation knob 6 and a side wall 7e of the cylinder 7. When the level of water staying in this space reaches the level of water in the concave 11b and exceeds the height of the side wall 7e of the cylinder 7, water flows through an upper opening 7b into the cylinder 7. Then, the water coming into the cylinder 7 passes through the lower opening 7c of the cylinder 7 and enters into the case 1. Bold arrows shown in FIG. 12 indicate the flow path of the water.
For overcoming this problem, it is considered that lengths A and B shown in FIG. 12 are widened. In this case, the discharge amount increases, thereby offering advantages to a certain extent. However, there is a limitation to widening the lengths A and B since design improvement and miniaturization considering the component arrangement within the case 1 are required. Thus, in the conventional switching device, water cannot be sufficiently discharged when a large volume of water flows into the concave 11b at a time and thus entrance of water into the case 1 cannot be prevented.
Nothing is described about prevention of water which enters from below the operation knob 6 in Patent Reference Nos. 1 and 2. Additionally, while Patent Reference No. 3 discloses a waterproofing technique for preventing short-circuit, entrance of water from below the operation knob into the case cannot be prevented in such a switching device which does not use rubber contacts according to the technique shown in this reference.